The temperate bacteriophage Mu is characterized by the highly promiscuous integration of its DNA into the genome of its host bacterium Escherichia coli. Covalent association of Mu with random host sequences is an integral part of the Mu life cycle. We are analyzing the life cycle of Mu by a combination of genetic, biochemical, and electron microscopic techniques. We wish to unravel the mechanism by which Mu DNA is inserted into the host chromosome and purify proteins which bring about this random nonhomologous integration. The mechanism by which Mu DNA is inserted into the host DNA is interesting not only because of random integration of Mu, but also because it is a model system for studies on transposition of DNA sequences.